The present invention relates to a semiconductor device with memory transistors. Particularly, this invention relates to a semiconductor device with memory transistors applicable to a gain-programmable amplifier, etc.
FIG. 1 shows a circuit diagram of a conventional differential amplifier with differential inputs and a single output for various analog signal processing, such as audio signal processing.
As shown in FIG. 1, input transistors 1 and 2 are connected to each other for differential amplification of input voltages Vine and Vine. The input transistors 1 and 2 are complementary conductance- and gain-variable devices. The sources of the input transistors 1 and 2 forming an input differential pair are connected together and further connected to a constant voltage supply Vas via a current supply transistor 5 as a control current supply device. In the drawing, Ice denotes a supply current.
The drains of the transistors 1 and 2 are connected to the drains of transistors 3 and 4, respectively. The transistors 3 and 4 form a current mirror circuit as an active load of the differential pair of the transistors 1 and 2. A current Iou is output from the drain of the transistor 2. The sources of the transistors 3 and 4 are connected together and further connected to a constant voltage supply Vd.
The entire circuit shown in FIG. 1 is assembled to an integrated differential amplifier.
The input and output characteristics of the differential amplifier depends on the supply current decided by the current supply transistor and the differential input voltages, and is expressed by EQU Iou.varies.(Vine-Vine+Vos).times.Ice
where Vos denotes a difference between threshold voltages of the input transistors 1 and 2 and is an input offset voltage of the differential amplifier.
The Japanese Patent Laid-Open Nos. 63(1988)-3503 and 4(1992)-192703 disclose circuitry with a transistor having a floating gate for varying the input and output characteristics of the differential amplifier described above.
Here, input and output characteristics of the differential amplifier are expressed by EQU Iou.varies.(Vine-Vine).times.Ice
and hence the input offset voltage becomes zero with no variation and signal fluctuation can be restricted.
However, in case of audio signals, a sound having a frequency of zero Hz (i.e., d.c.) cannot be heard. Hence, a direct current obtained by controlling an amplifier for amplifying an audio signal so that input offset voltage is zero is of no importance. In other words, the amplitude varying function is more important than the input offset voltage controlling function for audio signal processing.
For varying amplitude of an amplifier, gain programmable amplifiers with feedback circuitry have been provided. However, the feedback circuitry outputs a voltage that must be converted into a current. Therefore, this type of amplifiers require a voltage-to-current converter, and hence result in complex amplifier circuitry.